A laser-sculptured printing plate material is known which comprises a mixture of polyethylene and carbon black as molded in the form of a sheet. However, this material requires high-energy laser radiation for creating a concave-convex (three-dimensional) pattern on its surface. For this reason, any printing plate material on which a three-dimensional pattern can be produced with a low-energy laser radiation has not been commercially implemented yet.
Meanwhile, a recording material supplemented with nitrocellulose for enhanced sensitivity is known but this material is deficient in the strength required of any printing plate. By way of example, Japanese Patent Publication No. 51-35144 discloses an image forming technology which comprises coating a support with a photosensitive composition containing nitrocellulose and carbon black and irradiating the coated support with laser light from behind the support to form an image. Described in this prior art literature is an embodiment in which an ink is transferred to the photo-sensitive layer for printing. Moreover, Japanese Patent Application Laid-open No. 5-8367 discloses a gravure printing plate material having a photosensitive layer containing nitrocellulose and carbon black. However, these photosensitive layers are invariably of low strength so that the final materials are not practically useful as printing plates.
The strength of a printing plate material is not only dependent on the strength of the photosensitive layer but also on the strength of the support. For example, when a crystalline polyester film, for example a biaxially-oriented polyethylene terephthalate film, is used for the support, its strength varies according to processing conditions such as coating with a photo-sensitive composition. Thus, the inherent performance characteristics of the crystalline polyester are not fully expressed so that the printing plate may not be imparted with sufficiently high strength.
Furthermore, any printing plate material is required to have a high degree of surface smoothness in order to preclude background staining of the print due to residues of the printing ink on the plate surface. However, the surface smoothness of the printing plate manufactured by using a photosensitive composition containing nitrocellulose and carbon black is not so high. The rough plate surface is not conducive to soil-free prints.
Particularly when the printing ink is a water-based ink which is easy to handle and high in safety and hygienics, it is difficult to scrape off the excess ink from the printing plate with a blade even if the plate has a smooth surface, probably because the ink used has a higher surface tension than the gravure printing ink containing an organic solvent. The failure to clean the plate surface thoroughly with a blade leads to ink residues on the plate and, hence, stained prints.
Japanese Patent Publication No. 51-6569 discloses an image-forming printing plate material having a photosensitive layer containing nitrocellulose as a self-oxidative binder, carbon black as a particulate pigment capable of absorbing laser energy, and melamine resin as a curing agent (crosslinking agent). This literature describes examples in which an alkyd resin, a methyl methacrylate resin, a butyral resin, an epoxy resin or a novolac resin is further used. However, the photosensitive material described in the above literature is designed for use in the process for producing a lithographic or planographic printing plate which comprises contacting the photosensitive coating layer coated on a support with the polyvinyl alcohol coating layer coated on an aluminum plate and irradiating them with laser light from behind the support to thereby transfer an image corresponding to the irradiated area to the aluminum plate. Therefore, it is not a printing plate belonging to the category in which a printing ink is directly transferred on a photosensitive layer for printing. Moreover, the sensitivity and film strength of the photosensitive layer formed with the photosensitive material are not sufficiently high so that the product can hardly be used, as it is, as a printing plate.
Therefore, it is an object of the present invention to provide a printing plate material on which a concave-convex (three-dimensional) pattern can be formed even at a low laser energy output and which has a high film strength and a method of producing the printing plate material.
It is a further object of the present invention to provide a printing plate material havig a high sensitivity relative to laser light and useful to form a concave-convex pattern with high accuracy and a method of producing the printing plates material.
It is a still further object of the present invention to provide a printing plate material which can be used in a printing process comprising transferring a printing ink directly on the surface of its layer having a concave-convex pattern produced with laser light and has excellent strength, durability and wear resistance (printing pressure resistance) in association with printing and a method of producing the printing plate material.
It is still another object of the present invention to provide a printing plate material having a high degree of surface smoothness and inhibiting a background staining of the print and a method of producing the printing plate material.
A further object of the present invention is to provide a printing plate material which can be cleaned by scraping off an excess of printing ink from its surface by means of a blade even when a water-based printing ink is employed and, therefore, provides for a minimum background staining of the print and a method of producing the printing plate material.